EMC What is it and Where Did it Come From? Peter Garrahan NEKDA October 30-31, 2007

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1 EMC What is it and Where Did it Come From? Peter Garrahan NEKDA October 30-31, 2007

2 OVERVIEW OF PRESENTATION Basic Wood Science Investigative Work on Origins and Derivation of Tables Current Situation and Significance of EMC Tables

3 EQUILIBRIUM MOISTURE CONTENT In relation to the environment around MC usually defined based on oven-dry basis

4 EMC Is it a property of the air? Is it a property of the material?

5 Some Basics Wood (and other organic materials) is hygroscopic At any RH > 0% wood will have some moisture present Water can exist as free water or bound water Limited space (sites) available for bound water

8 FSP Fiber Saturation Point Achieved when all sites are full and no free water present. Sitka Spruce FSP = 27% ranges from 23 to 33% depending on temperature

9 Drying (i.e. Low RH high MC)

10 Wetting (i.e. High RH low MC)

11 In Equilibrium EMC)

12 Determining EMC Hold a piece of wood at fixed RH and temperature It will eventually reach equilibrium It will stop losing or gaining weight It s MC will now be equal to the EMC

13 Factors Affecting EMC Wood Species Wood Temperature Drying Temperature (air dry vs. KD) Drying vs. Wetting Heartwood vs. Sapwood

14 Species Effect on 50º F. & 75% RH Highest EMC = 15.7% Lowest EMC = 11.6%

15 Wood Temperature Effect on 50º F. & 75% RH - EMC = 150º F. & 75% RH EMC = 12.0%

16 Drying Temperature Effect on EMC Wood dried at high temperature will equalize at a lower EMC Thermal modification process results in better dimensional stabilization.

17 Effect of Drying vs. Wetting on EMC - hysterisis effect

18 Effect of Heartwood and Sapwood on EMC Effect of chemical constituents (extractives) Lower EMC values for heartwood

19 U.S. FPL Tables on EMC

20 Work by Stamm and Loughborough Exclusively on Sitka Spruce - why? Relatively small number of samples equalized over a range of temperature and relative humidity Resulted in graphic representation

22 1949 Keylwerth Extrapolated Data to Temperatures Over Boiling Extended range of data produced by Stamm

24 By the 1950 s we had EMC Tables That Were in the Wood Handbook and the Dry Kiln Operator s Manual These tables were generated by visual inspection of the graphs to look up values at various intersection points of DB temperature and RH or WB

25 Starting in the 1970 s Bill Simpson of the U.S. FPL analyzed data in EMC tables to develop a mathematical model:

26 By 2007 Peter Comes Along and Needs to Find an EMC Table for his new Kiln Operator Manual Which one to use: U.S. original version? U.S. version with Hi Temp data? U.S. version determined by Simpson s model? Canadian version of tables from 1940 s?

27 Purpose of EMC Table in a Kiln Manual Compare drying conditions Relate one schedule step to another Establish potential for MC gradient

28 In kiln drying we only use EMC as a measure of the relative ability of the air to dry the wood further More important to standardize with a specific table rather than try to use species specific values Different tables in different manuals only creates confusion.

29 New tables are based on Stamm s work and as expressed mathematically by Simpson Tables have been compressed to cover range of temperature and depression on a single page Version of table in both Fahrenheit and Celsius

30 Thank You

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